Inductive reasoning in the courtroom: Judging guilt based on uncertain evidence

Ann Martin, University of New South Wales

Brett Hayes, University of New South Wales

Abstract

Most legal systems require jurors to consider all the evidence
presented at trial. Hence when there is uncertainty over aspects of evidence this
should be factored into juror judgments. Two experiments examined how mock jurors
used uncertain information in their ratings of defendant guilt and final
verdicts. Participants read scenarios where an eyewitness expressed uncertainty
about the identity of a critical piece of evidence (e.g. the object a defendant
was holding could have been a knife or a mobile phone). The respective
probability of these alternatives was varied, as was their association with the
alleged crime. When the probability of the alternatives was varied between
subjects (Experiment 1) there was only weak evidence that jurors considered both
alternatives. When probability was varied within-subjects (Experiment 2), jurors
did consider both alternatives in their guilt judgments. The implications for
theories of reasoning with uncertain information and forensic practice are
discussed.